The present invention relates to quick service valves for use in automatic fluid pressure brake systems for railway trains, and particularly to such quick service valves as provide "continual" quick service operation in response to successive reductions in brake pipe pressure during a service brake application.
The well-known, standard ABDW control valve device used on freight-type railway cars to control the car brakes in response to variations in the train brake pipe pressure employs a quick service valve portion that provides the aforementioned "continual" quick service function. Unlike other "continual" quick service type valves, this ABDW quick service valve is piloted by the quick action chamber "breathing" pressure developed by the action of the ABDW valve emergency piston in providing stability against undesired emergencies during service brake applications. While the local venting of brake pipe pressure realized by the quick service activity of the ABDW control valve device is adequate under normal conditions, where unusually long runs of brake pipe occurs between these ABDW control valves, such as in intermodal, multi-pac, and other specialty-type car arrangements, a need exists for an additional quick service valve device that can be employed independently of the car control valve to supplement the local quick service action of the ABDW control valve in order to assure propagation of the brake pipe pressure reduction wave during service braking. Continual quick service valve devices known in the prior art are cyclic in operation, generally employing at least two separate valves to alternately vent brake pipe pressure locally in response to a reduction in the trainline brake pipe pressure and, subsequently, to vent a reference pressure with which the brake pipe pressure is compared across a control piston to terminate the local venting of brake pipe pressure for a time duration sufficient to assure that a subsequent cycle of operation can only result from a continuing reduction of the trainline brake pipe pressure and not from the local venting of brake pipe pressure.
Not only are these known prior art valve devices inefficient in operation, due to the absence of any quick service venting of brake pipe pressure during the "off" period of each cycle of operation, but perhaps more importantly, when used to supplement the ABDW quick service function, their cyclic operation tends to cause the ABDW quick service valves to stall out, since the "breathing" pressure that pilots the ABDW quick service valve operation tends to dissipate during the "off" period of each cycle. Accordingly, propagation of the pressure reduction wave through the train brake pipe can become degraded rather than improved by these known quick service valves when used to supplement an ABDW type quick service valve.